Microscopes comprise at least two imaging optical systems, one main objective and an eyepiece. In principle, all optical observation devices have the same fundamental structure of objective and eyepiece. These two optical elements, objective and eyepiece, form a total system and predefine both the possible magnification range and the field depth range. In many areas where microscopes are used, in particular surgical microscopes such as those used in neurosurgery, a very high degree of depth of field is important, because the surgeon has to see not only the focusing plane, but also as many regions of the object as possible in a range of sharp focus.
Improvement of depth of field is traditionally achieved by decreasing the size of the aperture. There are stereomicroscopes, such as the LEICA M651 and LEICA M690 (brochure LEICA M651/M690 X.96-SCH 1995, page 6), which provide a double iris diaphragm for simultaneously decreasing the aperture of both beam paths. The disadvantage that has to be accepted with this traditional way of improving the depth of field, however, is the loss in terms of resolution and image brightness.
A further, albeit technically very complicated increase in depth of field is known from EP-B1-0988572. Here, provision is made for a device which modifies the focal length of the system at very high speed. This is done either by displacing a lens longitudinally with respect to the beam path or by placing a lens having different optical properties transversely into the beam path. A shutter opens the beam path in cyclic fashion in relation to this movement only when the movement has reached its outermost point and briefly comes to a halt. The speed of this movement must be sufficiently high for so-called “stereoscopic pumping” (image movement) and brightness flicker not to occur.
Quite apart from the very high technical outlay mentioned, this solution is not practical in microscopes, if only for weight and space reasons.